OE class refines tool for undersea mapping

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Students in MIT's Department of Ocean Engineering recently developed a variation of a sonar system used to map the ocean floor. Intended to be carried aboard an Autonomous Underwater Vehicle (AUV), the system will provide a cheaper and more flexible means of sub-bottom mapping.

A "sparker" was the project of the five students in 13.011, Ocean Science and Technology Laboratory, taught by research engineer Charles Mazel and research specialist Noah Eckhouse, both of the Department of Ocean Engineering. Compact enough to be contained in a small pressure housing (currently simulated by a plastic lunch cooler), the device generates a high voltage across a pair of electrodes in the water. The rapid vaporization of the water and expansion of the vapor bubble produce a short, powerful acoustic pulse. The acoustic energy penetrates the sea floor and reflects from sediment layers, underlying rock, faults and other topographical features. The echo is received by hydrophones and used to form a picture of the bottom that is useful for geological mapping.

Much larger versions of sparker arrays are in use on geophysical survey ships. Yet because of the amount of equipment and crew needed for those vessels, undersea mapping by that method is substantially more costly than using a small sparker aboard an AUV.

The compact version built by the students operates at much lower power than the ship-based systems (20 joules compared to thousands of joules), and consequently has a more limited range of penetration. However, many potential missions do not require deep bottom penetration; this, coupled with the lower operational costs of the 13.011 sparker, make the platform very desirable. For example, in an eight-hour AUV mission, the sparker and its related equipment could map an area large enough to accommodate an oil and gas platform, Dr. Mazel and Mr. Eckhouse said.

The class project, which originated with an idea by Assistant Professor Robert Fricke, elicited much enthusiasm and innovative thinking by both the students and instructors. "One of the goals of this course is to couple undergraduate teaching with original research and group problem solving," Mr. Eckhouse said.

"Noah and I have worked on many ocean-related projects, but the sparker was new to us as it was to the students," Dr. Mazel said. "Our job was to guide them through the design process, working together as a team. In a very short time they came up with good questions, then designed experiments to solve them."

The design goal posed to the students was to create a unit that could be deployed on the MIT Sea Grant Odyssey II AUV, without a specific mission in mind. During the term, some very real interest in the project emerged for specific applications in science and industry, and there may well be work to carry the development further. "The students have really gotten to see how projects evolve from idea to prototype, and how changing constraints require them to stay flexible," Dr. Mazel said.